Dialkyl ethers of polyalkylene glycols are well known in the gas treatment art as solvents for acidic gases such as carbon dioxide (CO2), hydrogen sulfide (H2S), carbonyl sulfides (COS), carbon disulfide (CS2), mercaptans, and the like. Typical of the numerous disclosures of such solvents in the art are U.S. Pat. Nos. 3,737,392; 3,824,766; 3,837,143; 4,044,100; 4,581,154; 4,741,745; and 4,946,620, among others.
According to such references, it is well known that the dialkyl ethers of polyalkylene glycols, and in particular, for present purposes, the dimethyl ethers, comprise mixtures of polyalkylene adducts typically ranging from 1 to about 9 units of the ethylene oxide-derived moiety, xe2x80x94(C2H4O)xe2x80x94. U.S. Pat. No. 4,581,154 is particularly informative in that regard. That reference discloses a bell curve distribution of xe2x80x94(C2H4O)xe2x80x94 homologs, with the peak at 50 weight percent of the 3-homolog and a maximum content of 40 weight percent of the 4-homolog. That reference further teaches that the most preferred composition, at least for low temperature use, has a distribution of the 3- and 4-homologs at 33 weight percent each.
While the compositions of the prior art are generally effective for their intended purpose, they are not optimal. More specifically, it would be desirable to have a gas treatment composition which provides an improved balance of volatility and viscosity, and also improves the removal vs. retention balance of at least one key acidic component of the gas being treated. The present invention responds to that need.
The present invention provides a solvent composition for removal of acidic gaseous impurities from gaseous streams bearing such impurities, comprising a mixture of dimethyl ethers of polyethylene glycols of the formula CH3O(C2H4O)xCH3 wherein x is from about 3 to about 9 and wherein in at least about 50 weight percent of the total dimethyl ethers x equals 4. In a preferred embodiment, the present invention provides a composition for removal of acidic gaseous impurities from gaseous streams bearing such impurities, comprising a mixture of dimethyl ethers of polyethylene glycols of the formula CH3O(C2H4O)xCH3 wherein x is from about 3 to about 6 and wherein in at least about 40 weight percent of the total dimethyl ethers x equals 4 and no more than about 15 weight percent of the total dimethyl ethers is a combination of the 3-mole ethoxylate and the 5-mole ethoxylate.
In addition, the present invention provides an improved method for treating acid gas containing at least some carbon dioxide with a solvent, comprising contacting in a contacting means (typically, an absorber) the acid gas with a solvent comprising a mixture of alkyl ethers of alkylene oxides including at least some of the 4-mole ethoxylate of dimethyl ether, wherein the improvement comprises increasing the concentration of the 4-mole ethoxylate of dimethyl ether by a sufficient amount to significantly reduce the amount of carbon dioxide exiting the contacting means. Moreover, the present invention provides a method for treating a gas stream containing acidic gases comprising contacting the gas stream with a circulating solvent stream comprising at least about 1% by weight of the solvent stream of the 4mole ethoxylate of dimethyl ether in the substantial absence of the 3-mole ethoxylate of dimethyl ether.
As used herein, the terms xe2x80x9cgas,xe2x80x9d xe2x80x9cacid gas,xe2x80x9d and the like, are intended to refer to natural gas, hydrocarbon gas, syngas, steam reformer-type gases, and any other gas containing acidic, gaseous components which are to be removed by treatment with a solvent. Also as used herein, the expression xe2x80x9csubstantial absencexe2x80x9d is intended to refer to a concentration which has no commercially significant effect on the ability of the solvent mixture to remove carbon dioxide under the operating conditions being employed.
The present invention provides its improvement over the prior art by increasing the concentration of the 4-mole ethoxylate of dimethyl ether present in the solvent being used to treat an acid gas, particularly an acid gas containing carbon dioxide. As will be illustrated in the examples below, it has been unexpectedly found that the 4-mole ethoxylate of dimethyl ether is uniquely effective in retaining carbon dioxide, i.e., in reducing the amount of carbon dioxide exiting the contacting means. Accordingly, the present invention is particularly useful in commercial operations wherein the separation and recovery of carbon dioxide is economically advantageous and carbon dioxide xe2x80x9cslipxe2x80x9d is therefore desirably minimized. Such a commercial operation would include, for instance, an ammonia plant where carbon dioxide separated in the contactor can be flashed off from the alkyl ether alkoxylate solvent and recovered for sale, used for urea production, and the like.
In accordance with the present invention, it has been discovered that the pure 4-mole ethoxylate of dimethyl ether (i.e., tetraethylene glycol dimethyl ether) has excellent properties for the removal of acid gases, especially CO2, from methane, natural gas, syngas and hydrocarbon gases in general. Its combination of excellent affinity for the acid gases, low viscosity and low vapor pressure, makes it superior to any of the other homologs in the series for use in acid gas treatment. By comparison, triethylene glycol dimethyl ether is relatively volatile (relatively high vapor pressure) and is therefore readily lost with the products exiting the acid gas contacting means. On the other hand, pentaethylene glycol dimethyl ether and higher homologs are undesirably viscous, resulting in various problems of reparation and recovery downstream of the contactor.
Although tetraethylene glycol dimethyl ether is particularly effective as a solvent when used alone, it will be recognized that well-known methods of preparation of dialkyl ethers of alkylene glycols do not ordinarily produce pure products, so as a matter of economic convenience, a certain amount of other homologs may be tolerated in combination with the tetraethylene glycol dimethyl ether. In any such combination, it is preferred that tetraethylene glycol dimethyl ether be present in a concentration of at least about 50% by weight, more preferably at least about 60%, and most preferably at least about 80% by weight.
It has been found particularly desirable to restrict the distribution of ethylene oxide homologs to the 3 to 6-mole adducts, such that the 4-mole adduct substantially predominates the composition. More specifically, it is desirable that the combined concentration of the 3-mole, 5-mole, and 6-mole ethoxylate homologs make up no more than about 15%, preferably no more than about 10%, by weight of the total dimethyl ether mixture. In particular, it is preferable that the 3-mole homolog make up no more than about 2%, more preferably no more that about 1%, by weight of the total dimethyl ether mixture.
For start-up of treatment facilities, it is desirable that a solvent as high in tetraethylene glycol dimethyl ether as possible be used. Alternatively, it is quite effective to add a solvent high in tetraethylene glycol dimethyl ether to the pre-existing solvent in operating facilities in order to raise the average concentration of tetraethylene glycol dimethyl ether. It is highly desirable to raise the circulating concentration of tetraethylene glycol dimethyl ether to at least about 50% by weight. Where the concentration of adjacent homologs has been restricted, as discussed above, a significant beneficial effect can be observed at concentrations of the 4-mole adduct at levels much below 50% by weight. Moreover, in the substantial absence of the 3-mole adduct, a beneficial effect can be seen at concentrations of the 4-mole adduct down to as low as about 1% by weight, although the concentration of the 4-mole adduct is preferably at least about 30%, and more preferably at least about 60% by weight of the soolvent mixture. In an ideal composition, up to 100% by weight of the solvent can be the tetraethylene glycol dimethyl ether. It has been found, however, that even relatively small increases in the concentration of tetraethylene glycol dimethyl ether are surprisingly effective in reducing the amount of CO2 lost at the outlet of the contactor. Similarly, it has been surprisingly found that by increasing the concentration of circulating tetraethylene glycol dimethyl ether, the circulation rate can be substantially reduced while yet significantly reducing the amount of CO2 being lost. These facts are illustrated in the examples below.